Haloalkanes, and more particularly .alpha.,.omega.-bromochloroalkanes, are widely used as starting materials for the preparation of pharmaceutical, pesticidal and detergent products.
Many methods have been described for producing these .alpha.,.omega.-bromochloroalkanes.
They most often involve the reaction of halogens (bromine or chlorine) or of their derivatives, such as PBr.sub.3 or SBr.sub.6, with an .alpha.,.omega.-chlorohydroxyalkane or alternatively the reaction of halogens (bromine or chlorine) or of their derivatives, such as SOCl.sub.2, with a haloalkane or an .omega.-haloalkanoic acid.
British Patent 788,349 describes a process for the preparation of 1-bromo-4-chlorobutane which consists in treating, in a first stage, THF with dry hydrochloric acid in the presence of traces of ZnCl.sub.2 at a temperature which reaches approximately 100.degree. C. and then, in a second stage, in treating the 4-chloro-1-butanol obtained above with red phosphorus and then with dry bromine at a temperature of between 0.degree. C. and -10.degree. C. The 1-bromo-4-chlorobutane is obtained with a yield of approximately 62% with respect to the THF used.
U.S. Pat. No. 2,839,574 mentions a process for the preparation of 1-bromo-4-chlorobutane which avoids the use of red phosphorus and of bromine or of SBr6. This process consists in treating predistilled 4-chloro-1-butanol with dry gaseous hydrobromic acid in the presence of a solvent at boiling point which forms an azeotrope with the water formed according to the reaction: EQU Cl(CH.sub.2).sub.4 OH+HBr.fwdarw.Br(CH.sub.2).sub.4 Cl+H.sub.2 O.
The yield is approximately 70%.
In the Japanese patent application published under No. JP 5791930, the 1-bromo-4-chlorobutane was obtained with a yield of approximately 90% by treating freshly distilled 4-chloro-1-butanol with SBr.sub.6 formed from sulphur and bromine, according to the reaction scheme: EQU SBr.sub.6 +6Cl(CH.sub.2).sub.4 OH.fwdarw.6Br(CH.sub.2).sub.4 Cl+H.sub.2 SO.sub.4 +2H.sub.2 O
In the case where SBr.sub.6 is reacted with an unpurified 4-chloro-1-butanol, arising in particular from a mixture of tetrahydrofuran and hydrochloric acid, the 1-bromo-4-chlorobutane yield is approximately 70%.
D.C. Sayles and Ed. F. Degering (Journal of American Chemistry Society, 71, page 3162, 1949) describe a method for the preparation of 1-bromo-4-chlorobutane by treating n-bromobutane with sulphuryl chloride (SO.sub.2 Cl.sub.2) in the presence of benzoyl peroxide, at reflux of the reactants. The 1-bromo-4-chlorobutane yield is 35%.
Smushkevich, Yu.I et al. apply the Borodin-Hunsdiecker reaction to .omega.-chloroalkanoic acids (Tr. Mosk. Khim. Technol. Inst. No. 61, pp 47-48, 1969).
They thus obtain .alpha.,.omega.-bromochloroalkanes by treating .omega.-chloroalkanoic acids with HgO and bromine in CCl.sub.4 medium according to the reaction scheme: EQU Cl(CH.sub.2).sub.n COOH+Br.sub.2 .fwdarw.Cl(CH.sub.2).sub.n Br+CO.sub.2 +HB r
It should also be reported that Hahn, Roger C. (Journal of Organic Chemistry, 53 (6), pp. 1331-3, 1988) describes a method for the preparation of .alpha.,.omega.-bromochloroalkanes by a halogen exchange reaction. Thus, 1-bromo-6-chlorohexane is obtained by heating a mixture of n-bromobutane and 1,6-dichlorohexane in the presence of tetrabutylammonium bromide according to the reaction scheme: EQU Br(CH.sub.2).sub.3 CH.sub.3 +Cl(CH.sub.2).sub.6 Cl.fwdarw.Br(CH.sub.2).sub.6 Cl+Cl(CH.sub.2).sub.3 CH.sub.3
All these methods have many disadvantages. They use starting compounds which are often impure, requiring purification operations, and reactants which are expensive and difficult to handle (P+bromine, S+bromine).
The .alpha.,.omega.-bromochloroalkane yields are low and the products obtained contain impurities, the removal of which results in difficult and expensive separation operations.